1. Field of the Invention
The present invention relates generally to the production of a laminar stock material, one layer of which is a metal. In particular, the invention relates to a transfer process for the application of a thin, coherent metallic stratum, such as aluminum, to a major surface of a non-metallic substrate, such as paper, and to the product produced thereby.
2. Prior Art Statement
Over the years considerable effort has been expended in the development of stock material in the form of laminar webs, in order to achieve enhanced functional properties in addition to improved appearance. For example, laminates comprising a metallic layer have found widespread utility in the preparation of packages, overwraps, and the like, because of their lustrous appearance as well as the excellent moisture vapor and gas vapor barriers which they present. Of particular inportance in this field are the laminates comprising a thin metal layer and a fibrous layer, e.g., one fabricated from fibrous pulp, esp. paper. In this regard, the pertinent prior art against which the present invention stands out as an advantageous and non-obvious improvement is set forth as follows:
1. U.S. Pat. No. 3,730,752 discloses that a rough or porous material, such as uncoated paper, may be metallized by first being given a smooth surface by the application of a film of any of a number of plastic resinous priming materials. After solidification of this prime coat, a very thin layer of vaporized metal is deposited thereon. See esp. col. 2, lines 18-35.
By this process curl free products are very difficult to obtain. That is to say, the paper shrinks on the side from which water has been evaporated, resulting in a product with a high curl tendency. Moreover, the reflectivity of the products obtained leaves much to be desired. Coating of the rough surface of the paper is expected to provide a mirrorlike surface upon which the very thin layer of vaporized metal may be deposited. However, with thick precoatings on ordinary paper or thinner precoatings of very expensive, smoother papers, it is not possible to achieve a reflectivity of better than 50-70%, which is not satisfactory in today's market. Finally, the coatings employed cannot be brittle so as to cause a break upon folding of the final product. However, flexible plastic coatings change the dead fold properties of the final product, thereby introducing problems in the employment of most packaging machines in use today.
2. U.S. Pat. No. 3,043,728 discloses an apparatus and process for the vapor deposition of an aluminum layer upon a non-metallic substrate such as paper. A drum having a smooth, shiny surface is provided, upon which surface a release agent is deposited. The drum surface contining the release agent is then caused to rotate into a sealed, high-vacuum coating chamber, wherein this surface is exposed to aluminum vapor, which condenses thereon as a shiny film. Thereupon this surface moves out of the high-vacuum chamber and into contact with the adhesive surface carried by a paper substrate. The adhesive is firmly united to the thin aluminum film, after which the paper, with the aluminum film bonded thereto, is stripped from the drum, which then passes into a cleaning station prior to the application of another coating of release agent. See esp. col 2, lines 7-51. In the employment of this apparatus and process, residual release agent, specks of aluminum and/or adhesive are removed by brushing and grinding of the drum. Unfortunately, such treatment adversely affects the reflectivity of the final product, as it has a pernicious effect upon the smoothness of the replica surface of the drum. Moreover, synchronization of the individual steps of this in-line process is very difficult to achieve and maintain, as a result of which a considerable amount of waste product is produced. Finally, in order to prevent damage to the essential layers of the final laminar product (viz., the release coating layer and the aluminum stratum), such layers cannot make any contact with the seals of the high-vacuum chamber. This results in intolerable leakages from the chamber and extreme difficulty in the obtainment of acceptable vacuum levels therein.
3. West German Offenlegungsschrift No. 2555087 discloses a process for producing a shiny metallic coating upon a sheet or film such as paper. According to this process, the substrate sheet or film is coated on one surface thereof with a thin layer of a glaze, which upon subsequent removal of the vehicle therefrom is hardened or crosslinked after a certain time. Also supplied is a plastic film, upon one surface of which a metallization means is provided, wherein dispersed metal powder is integrated. The substrate and the plastic film are then brought together (as by rolling) so that the matallization means is absorbed by the glaze. The plastic film acts as an intermediate carrier during the drying, hardening, or polymerization of the glaze, whereupon the film is separated from the substrate, producing a substrate having a shiny metallic coating thereon. In the practice of such a process, it is necessary to have a high concentration of metal powder in the metallization medium, in order that the resulting metallic surface be sufficiently reflective. Unfortunately, such a high concentration of metal also results in a metallic surface having a readilly visible variation in lay. That is to say, an objectionable pattern may be easily seen in the surface. Furthermore, such as esthetically undesirable pattern is indicative of surface discontinuities (instead of the desired uninterrupted continuum), which do in fact result in inferior barrier properties. Furthermore, by this process it is very difficult, if not impossible, to prepare a final product having a metallic layer with a thickness of much less than 1 micron. This is indeed unfortunate, as the cost of useful metals such as aluminum continues to rise with the cost of the energy required for their production. To be expedient, this process would have to employ less than 10 percent of the aluminum or other metal currently required therein.